I'm sorry, I went through the process and thought that I was having a new problem unrelated to the u-boot upgrade but still in the same train of thought. Turns out that I had the "image_name" set to the wrong thing (no idea why) and that got me to the same spot as everyone else on this thread.

I haven't found an adequate howto for upgrading u-boot though. I can't get into the factory-shipped O/S (ubuntu) 'cause I can't remember the environment variables to do it, so I'm left with trying to do it with USB or TFTP. The howto I found though appear to be for an even older version of u-boot since it referred to using a command called "bubt" which doesn't appear to exist in the version I'm using. Can you point me to a good tutorial somewhere?